1. Field of the Invention
The present invention relates to boron steels for carburized gears, and more particularly to boron steels for carburized gears having an improvement in the heat-treatment distortion, surface oxidation in carburization and material cost, and having superior hardenability, mechanical strength and fatigue strength, over low alloy steels and other boron steels which are conventionally used for carburized gears.
2. Description of the Prior Art
As low alloy steels for carburized gears, there have been conventionally proposed uses of Cr-Mo steels and Ni-Cr-Mo steels containing carbon in an amount of about 0.20 weight %, taking into consideration of heat-treatment distortion, surface hardness, internal hardness and fatigue strength. However, Cr, Ni and Mo elements are rare elements which have a small estimated amount of deposits in the earth and thereby are expensive. Accordingly, the use of such expensive elements leads to the increase in the material cost of alloy steels for carburized gears.
For solving the above-mentioned problems, an attempt to use boron steels which were conventionally used for low grade parts of mechanical constructions has been made, by the applicant, in carburized gears. For example, the Korean Patent Application No. 90-19454 filed on Nov. 29, 1990 in the name of the applicant disclosed boron steels having an improvement in carburized gears. The boron steels disclosed in the Patent Application are steels for carburized gears which reduce the material cost by substituting boron for expensive nickel, chromium and molybdenum, have superior mechanical properties such as distortion in carburization, hardenability, strength and fatigue limit. However, they still have the problem of the surface oxidation in carburization which was encountered in conventional steels.
On the other hand, the surface oxidation phenomenon is caused by the fact that CO.sub.2 and H.sub.2 O in carburizing gas oxidized silicon, manganese and chromium in steel. Due to the oxidation of these alloying elements, the steel exhibits the reduced hardenability at its most surface layer. As a result, upon being subjected to a hardening, the steel forms a bainite structure distributed in the surface thickness about 20 .mu.m. This bainite structure results in poor hardness and tension stress at the surface of steel. The formation and the effect of surface oxidation is well known in this technical field. In order to eliminate the disadvantage caused by the bainite structure, the removal of the grain boundary oxidation is carried out by grinding the surface of gear. Alternatively, the gear may be subjected to a running-in process using a lubricating oil promoting the surface wear of the gear. However, since these methods are achieved with respect to the contact surface of gear, the grain boundary oxidized layer remaining at the tooth root portions of gear can not be removed. In particular, the surface oxidized layer remaining at the tooth roots of gear has been recently identified as the cause of gear tooth root breakage. Therefore, it has been strongly desired to provide a basic solution of reducing the formation of surface oxidized layer.